1. Field of the Invention
The present invention relates to a couple of gear wheels for driving a printing drum, and more particularly, a couple of gear wheels for driving a printing drum, comprising a first gear wheel mounted on the printing drum, and a second gear wheel rotatably supported by a part of a machine body, the first and second gear wheels being adapted to mesh with one another when the printing drum is set at a printing position located inside the machine body, so as to drive the printing drum by a drive source provided in the machine body via the meshing engagement the gear wheels, and to dissolve the meshing engagement when the printing drum is axially moved from the printing position toward outside the machine body with the first gear wheel carried thereby.
2. Description of the Prior Art
In Japanese Patent Laid-open Publication 59-12839, there is shown a drum unit of a rotary stencil printer, comprising a frame which supports a cylindrical printing drum having a perforated circumferential portion to be rotatable about its central axis, together with ink supply means for supplying ink to the printing drum from its inside, a receiver for an ink container for supplying ink, and an ink pump, while the frame is supported and guided relative to a machine body of the printer by support and guide means, to be movable along the central axis of the drum, so that the printing drum is movable between the printing position inside the machine body and a draw-out position outside the machine body. When the printing drum of the above-mentioned drum unit is set at the printing position inside the machine body, a shaft member on the printing drum projecting from an end wall of the printing drum coaxially to the central axis thereof coaxially engages into a mating hole of a drive shaft provided on the machine body, so that the printing drum is driven by a drive source provided in the machine body. Ever since, as such an axially engageable and disengageable drive means between the printing drum and the drive source of the machine body, it has been more widely adopted to provide a first spur or helical gear wheel on the printing drum, while providing a second spur or helical gear wheel on the machine body to be driven by a drive source, so that the first and second spur or helical gear wheels are meshed with one another when the printing drum is set at the printing position, thereby driving the printing drum by the drive source through the meshing engagement of those gear wheels.
With respect to such a couple of engageable/disengageable gear wheels for driving a printing drum based upon the parallel meshing engagement between the spur or helical gear wheels on the printing drum and the machine body, respectively, in order to ensure that, when the printing drum is returned into the machine body after having been drawn out of the machine body for an inspection, maintenance or color change, the gear wheel on the printing drum is engaged with the gear wheel on the machine body at a correct relative rotational position therebetween, it has been proposed by Japanese Patent Laid-open Publication 8-39918 to provide a disk at the gear wheel on the printing drum as attached to an end wall thereof facing the machine body, while also providing a disk at the gear wheel on the machine body as attached to an end wall thereof facing the printing drum, with notches being formed in those disks at a position rotationally corresponding to a predetermined relative rotational position between the two gear wheels, so that each of the two gear wheels can axially proceed across each disk of the other gear wheel only through the notched portion in order to enter into the mutual meshing engagement, thereby allowing a disengagement and a reengagement of the gear wheels on the printing drum and the machine body only when the gear wheel of the printing drum is positioned at a predetermined rotational position relative to that of the machine body.
Apart from the above-mentioned printing drum driving construction, in the rotary type stencil printers, a stencil leading end mounting device for clamping a leading edge of a stencil sheet to the printing drum along a generatrix thereof is provided in the printing drum so as to project radially outward from a cylindrical outer circumference of the printing drum, and in this connection, such an operation is required that a back press roller for pressing a print sheet against the rotating printing drum for applying a printing on the print sheet is retracted away from the printing drum when the stencil sheet leading end mounting device traverses in front of the back press roller, to avoid a collision of the stencil sheet leading end mounting device with the back press roller. When such a cyclic biasing of the back press roller is done by a cam adapted to rotate together with the gear wheel for driving the printing drum, it occurs in the rotational phase region where the cam presents a descending contour that the meshing gear wheels for driving the printing drum is inversely driven by the cam, so that the normally driven gear wheel advances relative to the normally driving gear wheel. If there is a substantial backlash in the meshing engagement of the driving and driven gear wheels, a noise will be generated by such a cyclic inversion of the backlash. In view of such a problem, it has been proposed by Japanese Patent Laid-open Publication 8-338500 to provide a gear sector as attached to a side portion of one of the gear wheels for driving the printing drum as a means for suppressing a reversal of the backlash.